Increasingly surgeons want to be able to custom fit humeral prosthesis to patients. Instead of having to choose a properly sized prosthesis from a group of preformed implants, it would be advantageous to have a basic design which can be modified with various component parts. This would eliminate the need to maintain a large inventory and would provide better fitting implants.
Custom fitted implants are particularly important in revision cases where an implant has to be removed and replaced. In this situation old cement must be removed and, in many cases, bone reabsorption occurs causing unpredictable proximal and/or distal bone loss or deformity which must be accommodated by the replacement prosthesis.
For initial implants, basic variations in patient anatomy are often confronted by the surgeon. Known humeral prosthetic implants do not achieve true anatomic compatibility with the proximal end of the humerus and do not provide the capability for modification of the humeral head size during surgery other than by replacement of the entire implant. Some patients have humeral necks that can be significantly longer or shorter than others. Also, shaft defects, which are not uncommon, are often not discovered until surgery. Variations in intramedullary canal diameter can also occur, which if not accompanied by a properly sized implant, can result in distal toggle. Currently available modular heads only fill the joint space of the glenoid cavity and not the proximal humeral bone cavity. Additionally, a patient's humeral anatomy can exhibit a wide proximal humeral cavity with a distal medullary canal that is half the size. Current humeral prosthesis do not allow for this type of mismatch.
Prior humeral prosthetic implants in use have a stem portion for implantation within the medullary canal of a human humerus and a head portion for engagement within the glenoid cavity of the human scapula. Such implants do not, however, achieve true anatomic compatibility with the proximal end of the human humerus, and do not provide a capability for modification of the stem during surgery other than by replacement of the entire implant.
A modular humeral prosthesis is known which is formed of a humeral stem and a head component which is readily attachable and detachable to and from each other for allowing intraoperative modifications of the implant. U.S. Pat. No. 4,919,607 entitled "Modular Humeral Prosthesis" issued to Dale et al. describes a modular prosthetic device which is anatomically compatible with the proximal human humerus and scapula and provides a modular head mechanism that can be interchangeable. U.S. Pat. No. 4,865,605 entitled "Modular Shoulder Prosthesis" issued to Dines et al describes a shoulder prosthesis which includes a humeral component and a glenoid component. These patents describe modular prosthesis which have only a head component and a stem component.
None of these modular humeral components address the problem of proximal humeral canal fill or variations in intramedullary canal diameter or length. This problem frequently occurs in revision cases wherein bone cement held an older prosthesis. The multiple part modular system of the present invention allows a surgeon to exchange parts intraoperatively to achieve optimal fixation of a humeral prosthesis, such as in revision cases wherein bone cement causes bone tissue removal upon removal of an earlier prosthesis.
The object of the invention therefore is to provide modular humeral prosthetic components which are anatomically compatible with the scapula, the proximal humerus and the distal stem portion of the humerus.
Another object of the invention is to provide a modular humeral prosthetic system of component parts which reduces the number of standard components and custom devices needed to achieve optimal fixation.